1. Field of the Invention
The present invention generally relates to an electromagnetic servo device. More particularly, the invention relates to an electromagnetic servo device of a type suitable for use in an electric type power steering system for vehicles.
2. Description of Relevant Art
In view of problems on the hydraulic type power steering system such as that the structure thereof was complicated, recent years have proposed a variety of electric type power steering systems for vehicles. In those electric type power steering systems were employed various types of electromagnetic servo device.
Exemplarily, in Japanese Patent Application No. 59-170812, filed on Aug. 16, 1984, there has been proposed by the present applicant an electromagnetic servo device for electric power steering systems for vehicles.
This electromagnetic servo device included an input shaft operatively connected to a steering wheel, an output shaft operatively connected to a road wheel to be turned, the output shaft being coaxial with the input shaft, a torque detection mechanism operatively connected to both the input shaft and the output shaft, an electric motor rotatably disposed around the output shaft, coaxially therewith, to supply auxiliary torque thereto, and a driving control circuit for controlling to drive the electric motor in accordance with a set of detection signals of the torque detection mechanism.
The torque detection mechanism outputs a pair of signals as the detection signals thereof representing the degree as well as the direction of a circumferencewise relative angular difference developed at the input shaft relative to the output shaft when steering torque was applied to the input shaft. The signals were fed to the driving control circuit, where they were processed to produce a torque magnitude signal and a torque direction signal, to send to the electric motor an armature current of a quantity according to the torque magnitude signal, in either direction of conduction thereof in accordance with the torque direction signal, whereby the output shaft was supplied with the auxiliary torque, while electromagnetic torque developed at the electric motor was transmitted as the auxiliary torque through a speed reducing mechanism to the output shaft.
Accordingly, the armature current to be fed to the electric motor had an increasing quantity, as the circumferencewise relative angular difference between the input and the output shafts was increased with steering torque applied to the input shaft.
In this electromagnetic servo device, the torque magnitude signal had a dead zone thereof controllable to be set.
In this electromagnetic servo device, therefore, the torque developed at the electric motor was transmitted as the auxiliary torque through the speed reducing mechanism in a smooth manner to the output shaft, whereby, for a driver of the vehicle, the burden that otherwise he or she had to bear when operating the steering wheel was adequately reduced, thus providing favorable steering characteristics.
However, in such an electromagnetic servo device, as well as other conventional ones for electric power steering systems for vehicles, there has been a desideratum to be achieved, such that an output shaft was desired to be rotated with good responsiveness, in dependence on the rotation of an input shaft, or more particularly, on the speed of rotation thereof.
The present invention has been achieved to accomplish such a desideratum in a conventional electromagnetic servo device.